WO2008117213A2

WO2008117213A2 - An assembly of at least two printed circuit boards and a method of assembling at least two printed circuit boards

Info

Publication number: WO2008117213A2
Application number: PCT/IB2008/051064
Authority: WO
Inventors: Koen Van Os; Marcel W. W. Coolen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-03-23
Filing date: 2008-03-20
Publication date: 2008-10-02
Also published as: WO2008117213A3

Abstract

An assembly of at least two printed circuit boards includes an intermediate connection layer (40) for providing electrical and mechanical connections between first and second circuit boards (10,12). The intermediate connection layer comprises a lower set of connection pads connecting the intermediate layer to the first circuit board, and an upper set of connection pads connecting the intermediate layer to the second circuit board. The intermediate connection layer comprises connection regions and a void region, wherein the void region extends between the first and second circuit boards and houses electronic components (16) of the circuit boards. This arrangement uses an intermediate connection layer to provide a stable mechanical connection, provide electrical connections between the circuit boards, and define a spacing for housing the electrical components of one or both of the circuit boards. The connections can be soldered, and this can be performed by reflow soldering methods, and using pick and place equipment.

Description

An assembly of at least two printed circuit boards and a method of assembling at least two printed circuit boards

FIELD OF THE INVENTION

This invention relates to circuit board assemblies, having multiple circuit boards. This invention further relates to a method of assembling circuit board assemblies, having multiple circuit boards.

BACKGROUND OF THE INVENTION

Many applications now use multiple circuit boards, connected together within small spaces, leading to 3D board constructions. Typically, dedicated printed circuit boards are used, which together combine desired specific functions. By way of example, a multiple board assembly can be created by providing a first printed board containing a power supply, a second circuit board containing a radio function and a third circuit board containing a sensor function. The three boards are then connected by means of electronic interconnects.

There are several ways to provide the connections between the boards. An example is to connect the boards by means of flexible connectors. These connectors are robust and flexible, but can require too much space.

SUMMARY OF THE INVENTION

There is therefore a need for a way to provide the required interconnects between boards but using a small space on the printed board. Furthermore, a robust mechanical interconnect system between the functional boards is also desired.

According to the invention, there is provided an assembly of at least two printed circuit boards, comprising: a first circuit board, which carries a first set of electronic components; a second circuit board, which carries a second set of electronic components; and an intermediate connection layer for providing electrical and mechanical connections between the first and second circuit boards, wherein the intermediate connection layer comprises a lower set of connection pads connecting the intermediate layer to the first circuit board, and an upper set of connection pads connecting the intermediate layer to the second circuit board, wherein the intermediate connection layer comprises connection regions and a void region, wherein the void region extends between the first and second circuit boards and houses one or more of the first set of electronic components.

This arrangement uses an intermediate connection layer to provide a stable mechanical connection, provide electrical connections between the circuit boards, and define a spacing for housing the electrical components of one or both of the circuit boards.

The connections can be soldered, for example performed by reflow soldering methods, and using pick and place equipment.

The electronic components typically comprise surface mount components. The first and second sets of electronic components can face each other, and both are then housed in the void. One or both of the first and second circuit boards can have electronic components on both sides. The invention can be used to stack and connect only two circuit boards, or it may be used for three or more circuit boards.

In one example, the intermediate connection layer comprises a set of discrete connections. These can be placed and soldered in the same way as other circuit components to the desired locations on the first or second circuit board. For example, each discrete connection can comprise a stack of a first and second checker component, with solder connection of the first checker component to the first circuit board, of the second checker component to the second circuit board, and between the first and second checker components. All of these operations can comprise standard pick and place and solder functions, and the checker components can be standard off-the-shelf items. Each discrete connection can instead comprise a single checker component, with solder connection of the checker component to the first circuit board and to the second circuit board.

In another example, the intermediate connection layer comprises an interface printed circuit board including at least one opening which defines the void. For example, the interface printed circuit board can comprise a set of plated vias for providing solder electrical and mechanical connections between the first and second printed circuit boards. The interface printed circuit board can also comprise a set of plated conductor tracks.

Preferably, the expansion coefficient of the interface printed circuit board is equal to the expansion coefficients of the first and second printed circuit board. The invention also provides a method of assembling at least two printed circuit boards, comprising: providing a first circuit board, which carries a first set of electronic components; - providing a second circuit board, which carries a second set of electronic components; and providing electrical and mechanical connections between the first and second circuit boards using an intermediate connection layer, by connecting the intermediate layer to the first circuit board at a lower set of connection pads, connecting the intermediate layer to the second circuit board at an upper set of connection pads, wherein the intermediate connection layer comprises connection regions and a void region, wherein the void region extends between the first and second circuit boards and houses one or more of the first set of electronic components.

This arrangement provides an assembly procedure which is very competitive because process steps can be integrated with conventional SMD (surface mount device) soldering. One step has to be added which will does require special tooling or equipment.

Embodiments of the invention will now be described, purely by way of example, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings: Fig. 1 shows a first method of forming a multiple board assembly in accordance with the invention;

Fig. 2 shows in more detail the checker components used in the method of Fig. i;

Fig. 3 shows variations to the assembly created by the method of Fig. 1; Fig. 4 shows a second method of forming a multiple board assembly in accordance with the invention;

Fig. 5 shows in more detail some of the components used in the method of Fig. 4; Fig. 6 shows in more detail the intermediate board sed in the method of Fig. 4; and

Fig. 7 shows variations to the assembly created by the method of Fig. 4. The dimensions of the diagrams are not to scale and like reference numerals refer to like elements throughout.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides an assembly of at least two printed circuit boards and an assembly method, in which an intermediate connection layer provides electrical and mechanical connections between circuit boards.

In a first example, the intermediate connection layer comprises a set of discrete connections.

Fig. 1 shows a first method of forming a multiple board assembly in accordance with the invention using discrete connections, which in this example are in the form of checker components. These are off-the-shelf components used for providing contact pads for signal checking during test and repair operations. These are of interest as they are readily available and can be mounted by conventional pick and place machinery as part of the mounting of the other functional components of a circuit board.

The checker components are provided between the functional boards. In Fig. IA, two circuit boards 10, 12 are shown. Solder paste 14 is applied by means of solder paste printing. Other techniques for applying solder paste can be used. Furthermore, conductive glues may instead be used.

In Fig. IB, the required circuit components 16 are placed with pick and place equipment. The checker components 18 are also supplied as additional SMD (surface mount device) components. The components 16 can include surface mount components, through- hole components and others. The placement of the checker components is as fast, as accurate and as cheap as placement of the other components.

In Fig. 1C, the two populated boards are reflow soldered. Misalignment problems due to incorrect positioning during pick and place (in step B) are corrected by the solder process, during which surface tension in the liquid solder material attracts components to their desired locations.

In Fig. ID, solder paste 20 is provided on top of the checker components 18 of one of the functional circuit boards. This can be implemented by a dispensing process or printing or by flux dipping (when pre-solder applied checker components are used). Several options are available to complete this step.

The two boards with corresponding checker component configurations (mirrored) are placed on top of each other in Fig. IE. This process can be done by means of special equipment, or standard flip chip pick and placement machines. Even manual positioning (optionally with the help of special jigs) can be implemented.

The final step of Fig. IF is a second reflow step. In this step, the solder will melt and the two checker components 18 will be connected to each other. If misalignment is introduced during this step, it can be corrected by the self-alignment effect with occurs during soldering automatically.

The assembly of Fig. IE has several advantages. The occupied area for making the interconnections between the boards is very small. Only the foot shape of the checker components is required to make the interconnections. This is normally much smaller than the space required for connectors. Also the positioning of the checker components can be designed in an arbitrary manner, so that the positions can be designed around all other required component positions. They may be positioned at the board periphery to give highest strength to the final assembly, but this is not required for electrical reasons. Indeed, electrical connections can be designed across the area of the two circuit boards being connected.

The gap between the functional boards can be controlled by the shape of the checker components 18. Typically the spacing will be between 0.3mm and 1.0 mm.

The checker components provide electrical and mechanical connections between the first and second circuit boards. The use of discrete connection points means that a void region is defined elsewhere which extends between the first and second circuit boards. The other circuit components extend into this void, so that the overall thickness of the assembly does not need to be increased by the presence of the checker components.

This open void structure also improves heat dissipation during the reflow soldering. The process window for reflow soldering is thus very broad.

The functional boards can be designed according to standard printed board layout design rules. This makes the design of the interconnects flexible, cheap and easy to implement. The method can be used to stack different size functional circuit boards.

The checker components are widely commercial available. One such component is shown in more detail in Fig. 2, and comprises a stainless steel body with solder plating 24. Fig. 3 shows various configurations of 3D stacked printed board application using the checker components for the interconnections, and shows the flexibility of the approach.

In Fig. 3 A, a stack of three printed circuit boards is shown, and with SMD components mounted on both sides of the middle board.

In Fig. 3B, a single checker component array is provided between the printed circuit boards. In this case, only one board is provided with the checker components. This is appropriate if the desired spacing can be obtained with a single checker component.

In Fig. 3C, the boards have different sizes. One board can also be flexible. In this example, the lower board 30 can be a flexible circuit board, which gives more flexibility in the mechanical design. The boards can also have different thickness.

In Fig. 3D, the void is used to cover a sensitive component by means of a robust cap.

In a second example, the intermediate connection layer comprises an interface printed circuit board including at least one opening which defines the void.

Fig. 4 shows a second method of forming a multiple board assembly in accordance with the invention using an interface circuit board.

In this example, an intermediate ("interface") circuit board 40 provides the electrical and mechanical connection between the circuit boards 10,12. In Fig. 4, the SMD components 16 of the lower board only 10 are shown.

As shown in the cross sectional drawing, there are copper plated vias 42 defined in the interface board 40. The circuit board 40 is defined as an annulus, with inwardly extending teeth 44 which are provided with the plated vias 42. The vias define the electrical connections between the functional boards above and below, in the same way as the checker components in the first example.

Solder paste is applied to either the interface board or the bottom board (by screen printing, plating or other dispensing technique), and the interface board 40 is then soldered to the bottom functional board 10. This can be done in an ordinary reflow soldering process in which the interface ring is handled like an electronic component. In a second reflow step, solder is applied on top of the interface board and the top functional board 12 is mounted on top.

This design shares the advantages of the first example above.

In addition, the teeth 44 of the interface board 40 can be made very small, and smaller than can be realised with discrete components. The use of an interface board means that only one component has to be placed in order to create multiple solder interconnects between the functional boards. This provides a simpler assembly procedure.

In the example shown, the interface board makes contact at the periphery of the functional board. This gives a robust design. The outer shape of the interface board can be very robust and can protect the more sensitive functional boards.

The interface board can be designed according standard printed board layout design rules. This makes the design and interconnect definition flexible, cheap and easy to implement. The expansion coefficient of the interface board is set to be equal to the expansion coefficient off the functional board. Therefore the structure will be not be sensitive to thermal gradients in the package.

As in the first example, the size and/or thickness of the functional boards can be different. Fig. 5 shows the three boards shown in Fig. 4 in more detail.

The left image shows the top view (component side) of the top board 12. The middle image shows the bottom board 10, and the right image shows the interface board 40, which is shown in more detail in Fig. 6, with the copper on the interface board shown as regions 60. The use of an intermediate circuit board gives additional flexibility to the overall design, as the circuit board can provide tracks so that the connections to the boards do not need to be at the same location above and below the interface board. In particular, the interface board layout is not restricted to plated through holes. The interface board can also be used to add additional functionality, for example electronic components or external interface circuitry.

Fig. 7 shows various configurations of 3D stacked printed board application using interface board for the interconnections, and shows the flexibility of the approach.

In Fig. 7A, an external interface 70 is provided which connects to one of the plated vias 72. In Fig. 7B, additional components 74 are mounted on the interface board, and these can connect to only one of the functional boards, so that plated vias do not have to be used.

In Fig. 7C, a stack of two interface boards 76,78 directly on each other can give even more solutions for the redistribution of the interface connections between the two functional boards. In the example shown, a combination of tracks and plated vias enables a staggered connection between the functional boards.

In Fig. 7D, the stack is extended by more than two functional boards and multiple interface boards. Two preferred examples have been shown. Those skilled in the art will appreciate that numerous additional versions may be implemented. For example, discrete connections may be implemented by SMD components which implement an electrical function, such as SMD resistors or capacitors (instead of checker components). Thus, the connections may form part of the circuitry function. The invention can use a wide variety of components as the intermediate connection layer, with soldering or other electrical connections to the circuit boards above and below, providing an easily automated electrical and mechanical connection as well as defining a desired spacing between the functional circuit boards.

Various other modifications will be apparent to those skilled in the art.

Claims

CLAIMS:

1. An assembly of at least two printed circuit boards, comprising: a first circuit board (10), which carries a first set of electronic components

(16); a second circuit board (12), which carries a second set of electronic components (16); and an intermediate connection layer (18,20; 40) for providing electrical and mechanical connections between the first and second circuit boards (10,12), wherein the intermediate connection layer comprises a lower set of connection pads connecting the intermediate layer (18,20; 40) to the first circuit board (10), and an upper set of connection pads connecting the intermediate layer (18,20; 40) to the second circuit board (12), wherein the intermediate connection layer comprises connection regions and a void region, wherein the void region extends between the first and second circuit boards and houses one or more of the first set of electronic components (16).

2. An assembly as claimed in claim 1, wherein the first and second sets of electronic components (16) face each other, and are housed in the void.

3. An assembly as claimed in any preceding claim, wherein one or both of the first and second circuit boards (10,12) has electronic components on both sides.

4. An assembly as claimed in any preceding claim, wherein the electronic components (16) comprise surface mount components.

5. An assembly as claimed in any preceding claim, wherein the connection pads comprise solder connection pads.

6. An assembly as claimed in any preceding claim, comprising at least a third circuit board.

7. An assembly as claimed in any preceding claim, wherein the intermediate connection layer (18,20; 40) comprises a set of discrete connections (18).

8. An assembly as claimed in claim 7, wherein each discrete connection comprises a stack of a first and second checker component (18), with solder connection of the first checker component to the first circuit board (10), of the second checker component to the second circuit board (12), and between the first and second checker components.

9. An assembly as claimed in claim 7, wherein each discrete connection comprises a checker component (18), with solder connection of the checker component to the first circuit board (10) and to the second circuit board (12).

10. An assembly as claimed in any one of claims 1 to 6, wherein the intermediate connection layer comprises an interface printed circuit board (40) including at least one opening which defines the void.

11. An assembly as claimed in claim 10, wherein the interface printed circuit board comprises a set of plated vias (60) for providing solder electrical and mechanical connections between the first and second printed circuit boards.

12. An assembly as claimed in claim 10 or 11, wherein the interface printed circuit board (40) comprises a set of plated conductor tracks.

13. An assembly as claimed in claim 10, 11 or 12, wherein the expansion coefficient of the interface printed circuit board is equal to the expansion coefficients of the first and second printed circuit board.

14. An assembly as claimed in any one of claims 10 to 13, wherein the interfaced circuit board (40) comprises a closed annulus.

15. A method of assembling at least two printed circuit boards, comprising: providing a first circuit board (10), which carries a first set of electronic components (16); providing a second circuit board (12), which carries a second set of electronic components (16); and providing electrical and mechanical connections between the first and second circuit boards using an intermediate connection layer (18,20; 40), by connecting the intermediate layer (18,20; 40) to the first circuit board (10) at a lower set of connection pads, connecting the intermediate layer (18,20; 40) to the second circuit board (12) at an upper set of connection pads, wherein the intermediate connection layer (18,20; 40) comprises connection regions and a void region, wherein the void region extends between the first and second circuit boards which houses one or more of the first set of electronic components (16).